This invention relates to a method of forming a laminate and an improved laminated core formed therefrom. More particularly, the invention relates to applying a liquid to the facing surfaces of laminations and applying sufficient pressure to the laminations to remove excess liquid and displace the air between the facing surfaces.
There are several applications such as electric power transformers, motors, electronics and catalytic converters employing thin gauge laminations. Thin gauge electrical steel or amorphous laminations for electrical applications reduce magnetically induced eddy currents by reducing the cross-sectional area through which those currents may flow. Grain oriented steel laminations have a thickness less than 0.5 mm, typically in the range of 0.18 to 0.35 mm. Amorphous laminations typically have a thickness of about 0.02 to 0.05 mm.
It is well known the above type electrical devices are more efficient when the thickness of the lamination is decreased with the lower limit for lamination thicknesses determined by manufacturing considerations. However, reducing the lamination thickness has undesirable effects on handling and fabrication productivity. Handling tissue-like thin laminations is a problem because the laminations are fragile and prone to damage during handling. The very thinness of the laminations reduces the productivity during processing and fabrication, making the product more labor intensive to utilize.
The prior art discloses adhesives, varnishes, oxides or mixtures thereof which may be applied to the surfaces of laminations so that several of the laminations can be bonded (or laminated) together for simultaneous processing. Processing a laminate greatly increases productivity and diminishes handling problems since the laminate is thicker and more rigid than a single lamination.
Nevertheless, there are several disadvantages when using adhesives, varnishes or oxides to bond laminations. To develop a good bond between laminations, the bonding agent normally must be applied as a relatively thick layer. Creating space between adjacent laminations which are wound or stacked into laminated articles is undesirable when used in electrical applications. This relationship of laminations and spaces involves a space factor which is the ratio of the volume of a stack of laminations under a given pressure to that of the solid material of the same mass. Thus, the space factor indicates the deficiency of effective volume due to the surface roughness of the laminations, lack of flatness of the laminations, or the presence of adhesive, coating, oxide and the like in between the adjacent surfaces of the laminations. A space factor of at least 90%, preferably greater than 95%, is desired for electric power transformers. Using a bonding agent that decreases the space factor is undesirable.
More recent prior art discloses adhesives which allegedly develop good bonds when applied as relatively thin layers and avoid decreasing the space factor below an acceptable level. However, a thin layer of adhesive tends to shrink when cured. Such a shrinkage, particularly for thin metal laminations may strain or induce stress into the laminations. Soft magnetic materials such as amorphous metal and grain oriented laminations are extremely sensitive to compressive stress. Induced compressive stress causes electrical power to be expended in the form of heat, i.e., core loss.
Another disadvantage when using chemical bonding is that the laminations become rigidly connected. If the laminate is wound into a coil such as around a mandrel having a small radius of curvature, the outer surface of a lamination must travel a greater distance than the inside surface of the lamination. Since the adjacent laminations are rigidly connected by a bonding agent, the laminations cannot move laterally relative to each other. As discussed above for chemically bonded electrical laminations, winding such a laminate into a coil may induce stress thereby increasing core losses of the laminate.
U.S. Pat. No. 4,277,530 teaches adhesive bonding of two or more sheets of electrical steel having thicknesses less than about 0.5 mm. The adhesive disclosed allegedly overcomes several of the problems associated with previously used adhesives. For example, a good bond is obtained between sheets when using thin glue lines so that a high stacking (space) factor can be obtained without inducing compressive stresses which act parallel to the rolling direction causing degradation of both magnetostriction and core loss characteristics. Instantaneous bonding is achieved at a curing temperature below about 400.degree. C. It is disclosed that adhesively bonded laminated articles: were less than 5% thicker than the total thickness of individual panels, showed an increase in core losses for electrical sheets of 5 to 16% above that for pairs of sheets before adhesive bonding, and the minimum roll radius to prevent damage when rewinding into a coil is 11 inches (280 mm).
Certain grades of electrical sheets are produced with reduced core loss achieved by inducing strain into the surface of a metal lamination such as thermal strain by rapid localized heating using a laser. The effect of thermal strain can be reduced if the laminations are exposed to elevated temperature. For example, a paper presented by Schoen et al. in October, 1986 to the ASM Materials Conference entitled Domain Refinement of Oriented Electrical Steel: From Early Beginnings to an Emerged Technology discloses that the effect of strain domain refinement is reduced when the strained metal lamination is exposed to temperatures of about 400.degree. C. or more. A further disadvantage when using chemical bonding is that heat may be required to cure the bonding agent. Curing may require additional heat treating equipment or a separate heat treating step prior to final fabrication of the laminate. Adhesives may require curing temperatures as high as 400.degree. C. while ceramic bonds may require temperatures as high as 900.degree. C. Consequently, the maximum benefit of this domain refinement technique cannot be preserved if the metal laminations are bonded together with an adhesive or ceramic requiring an elevated curing temperature.
Another disadvantage with chemical or ceramic bonding is that the bonding layers tend to be brittle. If chemically bonded laminations must be processed through cutting, punching or corrugating operations, the bonding layer may fracture causing the laminations to delaminate. Ceramic bonds may be broken by simply recoiling a laminate.
A laminate may be formed into cores by winding or punching and stacking with the cores used in an environment other than ambient air. For example, electric power transformers are frequently immersed in a dielectric cooling oil which may have a temperature of about 100.degree. C. The bonding agent must not only be stable at an elevated temperature but also must be chemically compatible with the oils over an extended period of time.
Accordingly, there remains a need for an improved technique for forming a laminate which can be easily handled without damage or delamination during fabrication. Furthermore, there remains a need for a laminate having no increase of spacing between adjacent laminations and a laminate whose laminations can remain free from stress or strain during fabrication.
I have discovered that a laminate can be formed and will resist separation indefinitely by applying a liquid of an appropriate viscosity to the facing surfaces of laminations. Sufficient pressure is applied to the laminations so that the facing surfaces are brought into intimate contact with each other and air between the facing surfaces is displaced by the liquid as excess liquid is removed from between the facing surfaces. The liquid remaining between the facing surfaces forms a seal preventing reentry of the air which enables the laminate to resist separation during subsequent processing and fabrication. The laminate formed has no increase in the space between its laminations and no induced stress in its laminations.